In many instances, portable or mobile measuring devices or imaging devices are used whose position has to be changed, depending upon the measurement being taken. Some of these devices change their behavior depending upon their alignment to the field of gravitation of the earth, e.g., because of changes in the dimensions or shape of the materials used. This can result in incorrect measurements or erroneous images. An accurate measurement of the line of the field of gravity of the earth in relation to the alignment of the measuring device is necessary in order to correct or to compensate for these errors.
French Patent Application No. 2608271 to Thome discloses a device for determining the vector of the gravitational force. This well-known device contains a body made of lead on a thread that can move freely in the horizontal position and which is aligned in a field of gravity. Balls having pre-marked centers have been arranged along this thread in such a way that the alignment of the centers of the balls makes it possible to determine the vector of the gravitational force. However, this well-known device only makes it possible to determine a three-dimensional system of coordinates whose vertical axis is defined by the vector of the gravitational force.
U.S. Pat. No. 5,197,476 to Nowacki discloses a device for determining a stable three-dimensional system of coordinates. This well-known device includes a three-dimensional frame that is equipped with Infra Red LEDs (Light Emitting Diodes), two infrared cameras and a computer. The positions of the Infrared LEDs in space are detected by means of the infrared cameras and those positions are then stored in the computer. After the frame is removed, the device measures the position of an ultrasound probe that also is equipped with Infrared LEDs and that can be moved manually within the volume previously defined by the frame relative to the previously stored positions of the Infrared LEDs on the frame. Hence, this device is only able to determine a stable three-dimensional system of coordinates that has no relation at all to the vector of the gravitational force.
It is at this juncture that the present invention wishes to provide a remedy. The present invention addresses the problem of determining a vector aligned in a local field of gravity. This vector lies within a space, for example within the space of an operating room, that has objects that are aligned in space and in relation to a stable system of coordinates. By incorporating the device made in accordance with the present invention in a CAS System (Computer Assisted Surgery System) a system of reference coordinates for measuring objects in medical robotics or in computer assisted surgical navigation can be established with an axis that runs parallel to the gravitational force.